Thermostat



March 6, 1956 A. J. HILGERT 2,737,555

THERMOSTAT Filed May 12, 1953 2 Sheets-Sheet l March 6, 1956 A. J. HILGERT THERMOSTAT 2 Sheets-Sheet 2 Filed May 12, 1953 United States Patent THERMOSTAT Adolph J. Hilgert, Milwaukee, Wis., assignor to Milwaukee Gas Specialty Company, Milwaukee, Wis, a corporation of Wisconsin Application May 12, 1953, Serial No. 354,444 11 Claims. (Cl. 200-140) This invention relates to improvements in condition responsive circuit controlling devices and more particularly to room thermostats.

High sensitivity and low differential are two qualities which are most desirable in condition responsive control devices. However, the voltage in the circuit being controlled by the device is usually of a magnitude to cause arcing at the contacts during making and breaking of the circuit therebetween. In order to minimize arcing, the air gap between the contacts must be relatively large, and the engagement and disengagement of the contacts must take place with a snap or quick-make, quick-break action. The resultant relatively large contact movement necessarily increases the differential of the device. Any friction between relatively movable parts of the device also contributes to its differential and impairs the sensitivity thereof.

With the above in mind, it is a general object of the invention to provide an improved condition responsive circuit controlling device in which substantially all frictional contributions to the differential of the device have been eliminated, thereby providing said device with improved sensitivity characteristics.

Another object of the invention is to provide an improved device of the character described having a snap leaf for providing quick-make or quick-break of the circuit at the contacts, there being means for precisely adjusting the air gap at the contacts to the operable minimum, and thereby minimize the contributions of said contact movement to the differential of the device.

A further object of the invention is to provide an improved circuit controlling device which is temperature responsive and which is provided with an anticipating heater for supplying artificial heat to the temperature sensing element of the device during a heating cycle, thereby substantially reducing the differential of the device, there being means for adjusting the heating rate of said heater to the maximum consistent with maintenance of the desired control point and not too frequent cycling, thereby permitting maximum differential reducing action by said heater.

A further object of the invention is to provide an improved circuit controlling device of the character described wherein the temperature sensing element is a volatile fluid filled expansible and contractible bellows having a wall movable directly and continuously with expansion and contraction of said bellows, there being integral shoulder means on said bellows wall in direct engagement with the contact leaf, so that all movement of said bellows wall is transmitted directly to the contact leaf without any frictional losses.

Another object of the invention is to provide an improved device of the character described having spring loading means for the bellows, there being novel force transmitting and alignment means connecting said loading means and the shoulder means of said bellows without interfering with the coaction of the contact leaf with said shoulder means.

2,737,555 Patented Mar. 6, 1956 Another object of the invention is to provide an improved device of the character described which is so constructed that exposure of said device to abnormally cold temperatures cannot abnormally deform the snap leaf or cause the latter to take a permanent set.

A further object of the invention is to provide an improved device of the character described which is strong and durable, reliable in operation and which can be readily assembled and disassembled.

Other and further objects and advantages will appear as the description proceeds, reference being had to the accompanying drawings illustrating one complete embodiment of the invention, and wherein:

Figure l is a side elevational view of the improved circuit controlling device, part of the cover and other parts being broken away and shown in section;

Figure 2 is a plan view of the invention with the cover and temperature control knob removed, parts being broken away and shown in section;

Figure 3 is a transverse sectional view taken approximately along the line 3-3 of Figure l; and

Figure 4 is a transverse sectional view taken approximately along the line 4-4 of Figure 1.

Referring more particularly to Figure 1 of the drawing, the invention comprises a base plate or block 10 of rectangular configuration and on which the mechanism of the improved thermostat is mounted. The block 10 is adapted to be removably mounted on a mounting plate or bracket 11 of similar outline, the latter being adapted to be permanently fixed to a wall or the like, as by screws 12. A removable cover 13 of rectangular shape is adapted to fit over the plate 10 and bracket 11, as shown, and to enclose the mechanism of the thermostat in a manner well known in the art.

In the form of the invention selected for illustration, the improved thermostat functions as a temperature responsive single-pole-single-throw snap switch. A snap leaf 14 acts as the blade of said switch and is mounted in a transverse plane normal to the block 10 as shown, being fixed at one end to the transversely extending arm 15, of an angle bracket 16, as by a clip 17 riveted to said arm by a rivet 18 (see Figure 4). The bracket 16 also has an extent 19 projecting normal to the block 10 as well as an arm 20 overlying said block and fixed thereto as by screws 21 (Figures 1 and 2).

The snap leaf 14 may be shaped as shown in Figure 4, having an irregular central area cut away to provide a pair of spaced legs 22 and 23, the end portions of which may be drawn together slightly and are clamped to the bracket 16 by the clip 17. The leaf 14 also has an intermediate tongue 24 which is shorter than the legs 22 and 23 and is formed with an aperture 25 adjacent its free end. As shown in Figures 2 and 4, the leaf 14 may be provided on opposite sides of its free end with integral flat contact members 26. As shown most clearly in Figure 2, the snap leaf tongue 24 curves outwardly at an angle to the plane of said leaf.

The snap leaf 14 is of the precision type which is characterized by the fact that said leaf requires an extremely small force on the tongue 24 for actuation from stable condition to unstable position. After the leaf 14 has snapped from stable to unstable position, the amount of retreat of the tongue necessary to cause the blade to reverse itself and return to stable position is a very short distance. One form of leaf 14 can be operated by pressures on the order of one ounce and will snap back to stable position from unstable position upon retreat of the tongue from actuated position a distance on the order of .0005 inch. Since only a small force and small movement is required to produce snap action of the leaf 14, the temperature sensing element is required to do only a relatively small amount of work in actuating said leaf, and the differential necessary to produce this snap action is correspondingly lowe red, as will hereinafter appear. he use of. the snap leaf N eliminates toggle or. like snap acting mechanisms ordinarily found in thermostats and which contribute to the differential of the device. At the same time, however, the snap leaf 14 eliminates slow-make, slow-break action ordinarily found in bimetal thermostats and which are subject to arcing and chattering.

A pair of spaced anglebrackets 27 and overlay the base block 11 adjacent the free end of the snap leaf and are fixed to the block as by screws 29 and 30. The brackets 27 and 28 have upstanding arinportions 31.

and 32'which are apertured to threadedly receive screws 33 and 34. In the illustrated form of the invention the rounded tip of the screw 33 electrically coacts with the contact 26 on leaf 14 in circuit-makingor circuit-interrupting relationship. The contact 26 ismovable with a snap action between the position shown, in which said contact engages the tip of the screw 33, and a retracted position in which said contact engages the tip of'the screw 34. In the form of the invention shown, the screw 34 serves as a stop member, and the threaded mounting of the screws 33 and 34' permit precise adjustment of the spacing between the tips of said screws so that the contact air gap can be maintained at a minimum value consistent with the limitations imposed by arcing conditions.

A U-shaped bracket 35 is fixed to the base block it), as by one or more screws 36, said bracket providing a rotatable support for a shaft 37 which extends normal to said block. The bracket 35 is formed with a downturned tongue 38, and a stop plate 39 is nonrotatably but axially slidably mounted on the shaft 37. The stop plate 39 has a cutaway peripheral portion providing a pair of angularly oifset shoulders 40 and 41 which are engageable with the tongue 38 to limit the rotation of the shaft 37. A cam 42 is fixed to the shaft 37 Within the bracket 35, and a coiled compression spring 43 surroundsthe shaft 37 between the cam 42 and plate 39 to urge the latter into frictional engagement with the bracket 35 and thereby tend to hold said shaft and cam in a selected position of rotation. One end of the shaft 37 projects through an aperture 44 in the cover 13 and carries a removable.

manually engageable dial or knob 45 which preferably bears temperature indicia (not shown) in a manner Well known in the art.

The bracket 35 has a bearing sleeve 46 fixed thereto and projecting parallel with the block 10, as shown; said bracket being apertured coaxially of. said sleeve. A cylindrical spring retainer 48 has an annular peripheral flange 49 at one end. thereof, said retainerhaving acoaxial stem projecting beyond said end, through thefibearing sleeve 46 andthe coaxial bracket aperture; A helical compression spring 59 coaxially fits on the cylindrical container 43, and one end of said springengages thefiange 49 as shown. The other end of the spring 50 coaxially engages the annular peripheral flange 51 of a spring 'retainer 52 which is spaced from the retainer 48 as shown. A stem or socket member 53 is fixed to and projects coaxially from the side of the retainer 52 opposite the spring 59. The stem 53 is formed with a bore 54 extending coaxially inwardly from the outer end thereof. A pin 55 is snugly positioned in the bore 54- and is bottomed therein, said pin also extending through the aperture 25 in the tongue 24 of the snap leaf 14, as is shown most' clearly in Figure 4.

The base block 10 is formed with an upstanding lug 56 near the end opposite the bracket 35, said lug providing a threaded mounting for a factory range adjustment screw '57 having a lock nut 58. A hermetically sealed expansible and contractible bellows 59 has a pair of relatively movable opposite end walls 60 and 61. formed with a coaxial recess or indentation 62 (Figure 1 for receiving the end of the range adjustment screw 57 The end wall 61 is as shown. The end wall 60 of the bellows is axially apertured, and a stem or socket member 63 is sealingly, and coaxially fixed in said aperture for movement with the wall 60. Th end of the stem 63 within the bellows is adjacent but spaced from the portion 62 of the end wall 61. The stem 63 has an opposite end face 64 which is formed with an axial bore 65. The bore 65 snugly receives the other end of the pin 55 in bottoming relationship therein.

it is apparent that the pin 55 serves not only as a force transmitting member transmitting forces between the loading spring 50 and the bellows 59, but that said pin also serves as an aligning means maintaining the spring and bellows assemblies in coaxial alignment. The relative positions of the contact leaf 14 and bellows 59 are such that the tongue 24 of said leaf continuously engages the shoulder provided by the end face 64 of the socket or stem 65, as shown.

It will be noted that in the improvedcircuit controlling device the factory range adjustment screw is remote from the force transmitting, connection between the loadingspring 5%); and the bellows 59; Turning the screw57 moves the wall 61 of the bellows and thereby shifts the axial position of said bellows to vary the force exerted: thereonby the spring; This adjustment, itwiil be noted, is' in'depend ent of the normal rangesetting mechanism including thecam 42, and it can be readily accomplished because of the conveniently accessible location of the screw 57.

The, bellows 59 is filled withv a thermally expansiblc and contractible volatile fluid fill. It is preferable that a subatmospheric type fill be used'so that, in the event of aleak in the bellows, the latter will expand and thercby provide fail-safe operation as will become apparent hereim after; Reference is hereby made to the copending-applications of Adolph J. Hilgert, Serial No. 265,445, filed- January 8; 1952, and Serial No. 291,047, filed May 31,

1952; and the application of John A. Wolff, SerialNo. 266,626, filed January 16, 1952. The referred to copending applications mention a number of volatile fiuid fills which are satisfactory for bellows use. It is under-stood, of course, that the particular fill selected dependsupon thetemperature range in which the bellows is expected'to operate. Illustrative of the sub-atmospheric fills mentioned in the Wolff application are ether, isopentane, isopropyl alcohol, normal propyl alcohol, andisobutyl alcohol.

Below the bellows 59, the base blockit is recessed to removably receive an anticipater assembly 66 which comprises an insulating base plate 67 having a pair of spaced eiectrically conducting plates 65' and @fi-xed thereto,- as by rivets 7i, and 71 respectively. The plate 68 has a laterally and upwardly projecting arm 72 which is electrically connected to the portion 19 of the bracket 16;. asby a screw 73; The plate 63 projects transversely inwardly', and has an upwardly offset which carries one end of strip'7 (Ft we 3) which is wound with wire 74' (Figures l and 2) having arelativci high resistance, said plate being el'ectricaliy connected to one endof said coil. The plate 6*)" is formedwith an armiS-which cxtends transverscly inwardly and'is shaped similarly to the portion 63' of the. 68, the ar-rn- 75 carrying the other end of the strip and being electrically connected to the other end of the coil-74'. The platecontrol circuit, for example a 24 volt circuit, by means 'ntothe brackets The screws and fitiimay en of screws Wand Stlwhiclr are threaded 27 and28 respectively. tend through the base block if: and. into themounthtg bracket or plate 11 to reinovably mount said block on nner end portion member said plate. With the parts of the thermostat disposed as shown in Figure 2, an electrical circuit is completed between the screws 79 and 88 via the bracket 27, screw 33, blade 14, bracket 16, plate 68, resistance coil 74, arm 77, plate 69 and its arm 76, and bracket 28. Normally the current flows through only that portion of the coil 74' between the plate 68 and the arm 77. However, if for some reason the arm 77 has a poor electrical contact with the coil, the current flows through the entire coil 74 directly to the plate 69 and its arm 76 Without passing through the arm 77. Thus, current flowing through the improved thermostat must flow through part or all of the coil 74, said current flowing through the anticipator coil 74 only when the contacts are in the closed position shown.

The amount of compressive force exerted on the bellows by the loading spring 50 is readily adjustable by rotation of the temperature control knob 45 and hence of the cam 42. This provides a convenient means for selectively varying the temperature control point of the improved thermostat.

When the improved device is connected in the control circuit of a furnace or the like, current flows through the coil 74, upon closure of the contacts, and causes said coil to heat up and raise the temperature of the bellows 59 by an amount slightly more than the amount of the operating differential of the device. In the absence of an anticipating coil such as the coil 74, once there has been a call for heat, the heating device would operate to heat up the furnace before heat is supplied to the space surrounding the thermostat and will continue to raise the furnace temperature while the space is being heated to actuate the thermostat through its operating differential. Then when the thermostat is satisfied, unwanted heat will be supplied to the space before the furnace cools down.

The amount of heat supplied by the coil 74 is directly related to the differential of the thermostat, the differential being decreased by an increase in the heat produced by the coil 74. The amount of heat produced by the coil 74', and hence the differential of the improved thermostat, can be readily adjusted by moving the contact arm 77 along said coil. The amount of anticipator heat is critical, because too much heat will undesirably reduce the control point of the thermostat during cold weather when continued operation of a furnace is required, since the coil 74' will be energized a large portion of the time. In mild wether too much anticipator heat results in undesirably short heating periods, i. e., too frequent cycling. Too little anticipator heat, on the other hand, raises the differential of the thermostat and will cause the heating system to overshoot in mild weather.

More specifically, the operation of the improved device is as follows: During a heating cycle the bellows 59 becomes heated by the'anticipating coil 74', and the volatile fluid fill in said bellows expands and moves the wall 60 and stem 63 toward the leaf 14, said expansion being opposed by the force of the spring 50. A rise in the temperature of the bellows to that set by the knob 45 and the cooperation of the cam 42 with the spring 50, causes expansion of the bellows and movement of the stem 63 and hence of the snap leaf tongue 24 to the left (as viewed in Figures 1 and 2) sufficiently to snap the free end of the leaf 14 to the right from its normal stable position in contact with the screw 33 to an unstable position in engagement with the screw 34. Disengagement of the leaf 14 from the screw 33 stops all current flow through the improved thermostat, and as a result, the anticipator coil 74 cools to permit the bellows 59 to sense the temperature of the circumambient atmosphere.

If the temperature of the circumambient atmosphere is below that set by the knob 45 and the cooperation of the cam 42 with the spring 50, the bellows 59 contracts under the influence of said spring, moving the stem 63 to the right (in Figures 1 and 2) and permitting the tongue 24 to retreat therewith to the point where the leaf 14 snaps back to a stable position in engagement with the screw 33. This, of course, again completes the electrical circuit through the thermostat in the manner previously described.

The small force and small movement required to actuate the precision snap leaf 14 requires only a relatively small temperature differential at the bellows 59 to snap said leaf from one position to the other. It will be noted that the bellows 59 acts directly on the snap leaf tongue 24 through the stem 63, there being no pivotally mounted force transmitting lever arms, toggle mechanism, or other friction producing elements such as are usually found in thermostats and which impair the sensitivity and contribute to the differential of said thermostats. The sensitivity of the improved thermostat is therefore substantially enhanced by this direct-acting feature.

As pointed out previously herein, there are two adjustment means by which the differential of the instant thermostat can be set at the operable minimum. By adjusting the positions of the screws 33 and 34, the air gap at the contacts can be set at the minimum consistent with the arcing conditions at said contacts, thereby providing minimum contact movement. By adjusting the position of the anticipator contact arm 77, the differential of the instant thermostat can be minimized by providing the maximum anticipater heating rate consistent with proper operation.

The portion of the stem 63 within the bellows 59 cooperates with the portion 62 of the bellows end wall 61 to provide a stop means limiting the contractile movement of said bellows. Thus, if the bellows 59 is subjected to abnormally low temperatures, the snap leaf 14 cannot be deformed to the point where said leaf will take a permanent set as a result.

The form of the invention shown and described has been selected for the purpose of disclosure only and is not intended as a limitation of the scope or use of the invention. It is understood that various changes and modifications may be made without departing from the spirit of the invention, and all of such changes are contemplated as may come within the scope of the claims.

What is claimed as the invention is:

1. In a thermostat: a thermally expansible and contractible bellows having a stationary wall and having a movable wall which is moved directly and continuously by expansion and contraction of said bellows; a spring retaining member; a coiled compression spring having one end coaxial and in engagement with said spring retainer; a stationary abutment means in engagement with the other end of said coil spring; a socket member integral and coaxial with said bellows wall; a snap leaf having an apertured intermediate portion positioned to continuously engage said socket to be actuated directly thereby in response to expansion and contraction of said bellows, said direct engagement of said socket with said intermediate snap leaf portion avoiding substantially all frictional contributions to the differential of the device; and a force transmitting and alignment member connected coaxially to said retainer and projecting through the aperture in said snap leaf and having an end portion snugly received in said socket to transmit forces between said spring and movable bellows wall and to maintain said spring and bellows in coaxial relationship.

2. In a condition responsive circuit controlling device, the combination of: an adjustably mounted first contact; an a-djustably mounted stop member spaced from said first contact; a snap leaf having a contact portion positioned between and alternatively engageable with said first contact and said stop member and normally engaging said first contact, said leaf also having an inter mediate portion actuation of which to a predetermined position causes movement of said contact portion into engagement with said stop member with a snap action; condition responsive actuating means in continuous engagement with said intermediate portion of the snap leaf; said actuating means being responsive to a predeterm'i'n'ed change" in the condition to actuate said intermediate snap leaf portion to said predetermined position to thereby snap" the contact portion of the snap leaf out of engagement with said first contact and into engagement with said stop member, the adjustable mounting'of said first contact and said stop member permitting precise adjustment of the spacing therebetween and hence of the contact movement, so that the contributions of said snap leaf to the-differential of the device can be kept to aminimum,

3. In a thermostat, the combination of: An adjustably mounted first contact; an adjustably mounted stop member spaced from said first contact; a snap leaf having an intermediate portion'and having a cont-act portion normally engaging said first contact and movable with a snap action into alternative engagement with said stop member upon actuation-of said intermediate portion to a predetermined position; and a temperature responsive expansible and contractible bellows having a movable portion'which is moved directly and continuously by expansion and contraction of said bellows, said bellows being positioned to cause said bellows portion to'continuously engage said intermediate snap leaf portion, and'said bellows being responsive to a predetermined temperature to actuate said intermediate snap leaf portion to said predetermined position to thereby snap the contact portion of the snap leaf out of engagement with said first contact and into engagement with said stop member, the adjustable mounting of said first contact and said stop member permitting precise adjustment of the spacing therebetween and hence of the contact movement so that the contributions of said snap leaf to the differential of the device can be kept to a minimum, and the direct engagement of said bellows portion with said intermediate snap leaf portion avoiding substantially all frictional contributions to the differential of the device.

4. In a thermostat: a stationary contact; a snap leaf having an intermediate portion and having a contact portion normally engaging said fixed contact and movable with a snap action to a retracted position spaced from said contact upon actuation of said intermediate snap leaf portion to a predetermined position; a temperature responsive expansible and contractible bellows having a movable wall which is moved directly and continuously by'expansion and contraction of said device, said wall being connected to said intermediate snap leaf portion to cause 7 actuation of the latter in response to expansion and contraction of the bellows; spring means for exerting compressive stresses resisting said thermal expansion of the bellows; and stop means limiting the contractile movement of said movable wall to prevent damage to the snap leaf as'a result of exposure of the bellows to abnormally low temperatures.

5. In a thermostat: A stationary contact; a snap leaf having an intermediate portion and having a contact portion normally engaging said stationary contact and movable with a snap action to a retracted position space-d from said contact upon actuation of said intermediate snap leaf portion to a predetermined position; a temperatureresponsive expansible and contractible bellows having a snap leaf actuating portion movable directly and continuously with expansion and contraction of said bellows, said'bellows being positioned so that said actuating portion thereof continuously engages said intermediate snap leaf portion to cause movement of the latter directly and continuously therewith; the direct engagement of said actuating portion with said intermediate snap leaf portion avoiding substantially all frictional contributions to the differential of the device; and an adjustable output electric heater thermally associated with said bellows and adapted to apply artificial heat to said bellows to thereby reduce the differential of the device, the adjustability of theoutput of said'heater providing for the adjustment of the differential of the device.

6; In a thermostat: An adjustably mounted first contact; an adjustably mounted stop member spaced from said first contact; a snap leaf having acontact portion positioned between and alternatively engageable with said first contact and said stop member and normally engaging said'first contact, said leaf also having an intermediate portion actuation of which to a predetermined position causes movement of said contact portion into engagement with said stop member with a snap action; a temperature responsive expansible and contractible bellows having a snap leaf actuating portion movable directly and continuously with expansion and contraction of said bellows, said bellows being positioned so that said actuating portion thereof continuously engages said intermediate snap leaf portion to cause movement of the latter directly and continously'therewith; and an adjustable output electric heater thermally associated with said bellows and adapted to apply artifical heat thereto to thereby re duce the differential of the device, the adjustability of the output of the heater and the adjustable mounting of said first contact and said stop member each providing an adjustment whereby the dilferential of the device can be kept at a minimum, and the direct engagement of the snap leaf actuating portion of the bellows with the intermediate snap leaf portion avoiding substantially all frictional contributions to the differential of the device.

7. In a thermostat, the combination of: A thermally expansible and contractible bellows having a pair of opposite end walls relatively movable on expansion and contraction of said bellows; adjustable abutment means in engagement with one of said bellows end walls; biasing means remote from said abutment mean and connected to the other of said bellows end walls to bias said other end wall toward said first-mentioned end wall and thereby bias said first-mentioned end wall against said abutment means; manually operable mechanism remote from said bellows and connected to said biasing means for selectively varying the biasing force of the latter on said bellows to thereby vary the control point of the thermostat, the adjustability of said abutment means providing for factory adjustment of the position of said first-mentioned beliows end wall toward or away from said other end Wall to thereby vary the temperature range of the thermostat; and a snap leaf having an intermediate portion in continuous engagement with a portion of said first bellows wall for actuation by movement of said wall during expansion of said bellows.

8. In a thermostat, the combination of: A thermally expansible and contractible bellows having a pair of relatively movable opposite end walls; a helical compression spring adjacent and substantially coaxial with said bellows, the end of said spring adjacent said bellows being connected to the adjacent end wall of said bellows in force trans mitting relation; adjustable abutment means in engagement with the end Wall of said bellows remote from said spring, said means defining the position of said wall, and the adjustability of said abutment means providing for factory adjustment of the temperature range setting of the thermostat; manually adjustable abutment mechanism connected to and defining the position of the end of said spring remote from the bellows, said latter mechanism providing means for selectively varying the biasing force exerted by said spring on said bellows and thereby varyingthe control point of the thermostat; and a snap leaf having an intermediate portion in continuous engagement with a portion of the bellows wall adjacent said spring for actuation by movement of said wall during expansion of said bellows.

9. In a condition responsive circuit-controliing device: A condition responsive expansible and contractible actuator having a portion movable directly and continuously by expansion: and contraction of said actuator; and a snap leaf having a resilient intermediate portion and avenues tion and having an overcenter position in which said contact portion is spaced from said first position, said snap leaf being positioned so that the resilience of the intermediate portion thereof affords an inherent bias maintaining said portion in continuous abutting engagement with said movable actuator portion throughout the operating range of the device whereby to cause movement of said intermediate snap leaf portion directly and continuously with said movable actuator portion and movement of said snap leaf to said over-center position with a snap action on predetermined movement of said actuator portion against the bias of said intermediate snap leaf portion, the abutting engagement of said intermediate snap leaf portion with said movable actuator portion avoiding substantially all frictional contributions to the differential of the device.

10. In a condition responsive circuit-controlling device: A condition responsive expansible and contractible hermetically sealed bellows having a portion movable directly and continuously by expansion and contraction of said bellows; a snap leaf having a resilient intermediate portion and having a contact portion, said snap leaf having a normal position in which said contact portion is in a first position and having an over-center safety position in which said contact portion is spaced from said first position; said snap leaf being positioned so that the resilience of the intermediate portion thereof affords an inherent bias maintaining said portion in continuous abutting engagement with said movable bellows portion throughout the operating range of the device whereby to cause movement of said intermediate snap leaf portion directly and continuously with said movable bellows portion and movement of said snap leaf to said overcenter safety position with a snap action on predetermined expansion of said bellows, the abutting engagement between said intermediate snap leaf portion and said movable bellows portion avoiding substantially all frictional contributions to the differential of the device; and a condition responsive expansible and contractible volatile fluid fill for said bellows having a vapor pressure below atmospheric pressure throughout the operating range of the device, whereby on occurrence of a leak in said bellows the entry thereinto of atmospheric pressure causes expansion of said bellows and actuation of said snap leaf to said safety position, thereby affording fail-safe operation of the device.

11. In a condittion responsive circuit-controlling device: A condition responsive actuator having a portion movable directly and continuously in response to changes in the condition; a snap leaf having an intermediate portion and having a contact portion, said snap leaf having a normal position in which said contact portion is in a first position and said leaf also having an over-center position in which said contact portion is spaced from its said first position, said actuator being positioned so that. said actuating portion thereof continuously engages said intermediate snap leaf portion to cause movement of the latter directly and continuously therewith and to cause movement of said snap leaf to said over-center position with a snap action on predetermined movement of said actuator portion in one direction, the direct engagement of said actuating portion with said intermediate snap leaf portion avoiding substantially all frictional contributions to the differential of the device; and an adjustable output electric heater thermally associated with said actuator and adapted to apply artificial heat thereto and thereby reduce the differential of the device, the adjustability of the output of said heater providing for adjust ment of the diiferential of the device.

References Cited in the file of this patent UNITED STATES PATENTS 1,905,788 Bast Apr. 25, 1933 1,985,337 Bondurant Dec. 25, 1934 2,010,649 Spencer Aug. 6, 1935 2,044,729 Eggleston et al June 16, 1936 2,080,306 Dobler et al May 11, 1937 2,092,085 Riley Sept 7, 1937 2,448,273 Pearce Aug. 31, 1948 2,544,208 Woods Mar. 6, 1951 

